In providing wireless communication using radio frequency modulated signals, it is desirable to enable two systems in communication, such as a remote system and a centralized communication hub such as shown in the above referenced patent applications entitled “System and Method for Broadband Millimeter Wave Data Communication,” to operate tuned to a same RF channel with a certain amount of accuracy in order to provide for reliable acquisition of signal information communicated. Moreover, it is generally desired that the systems and methods utilized to provide this frequency synchronization be able to accommodate frequency drift, or other inaccuracies, associated with design tolerances of components, temperature variations, and/or aging of components as well as to efficiently utilize resources.
Solutions have been developed which attempt to lock up, i.e., automatic frequency compensation (AFC), a remote clock to either a carrier signal that is transmitted from the source or try to lock up to a recovered clock, something that is extracted from the data. For example, one solution has been to use a dotting pattern, i.e., particular bits are placed in the data stream at predetermined positions, in order to allow a remote communication system recover a host clock. A problem with either of these techniques is that they usually provide a very limited lock range, so overall performance due to aging and drift from temperature can be unreliable. As long as the system is properly initialized and running, it will generally operate acceptably within the designed lock range. However, after such a system has been in operation for an extended period of time, it can no longer compensate for drifts beyond some point. Accordingly, if the system requires restarting or reinitialization, especially after the components have aged and thus their operating parameters have drifted, the system may not be able to reacquire the clock signal.
Although it might seem straight forward to enlarge the window of the lock range in which such a system may operate, such an endeavor inevitably leads to a trade off in the frequency characteristics of the system. Specifically, when the lock range over which the clock signal, such as a dotting pattern or the carrier frequency, may be acquired remotely generally results in the increase in phase noise associated with the reference oscillator used, i.e., the effect of the error or noise associated with the frequency versus time characteristics of the oscillator are inversely proportional to the range of frequencies over which the oscillator is used to acquire the signal. Phase noise contributes to the RF carrier through the relation 20 log(RF out frq/base osc freq) At relatively high frequencies, such as the millimeter wave (mmwave) frequencies of the above referenced patent applications entitled “System and Method for Broadband Millimeter Wave Data Communication,” an even slight increase in phase noise can cause undesired results, such as increased bit error rate (BER).
Moreover, the phase noise characteristics may become even more important in systems utilizing certain relatively high frequency, such as millimeter wave, front ends, such as embodiments shown in the above referenced patent application entitled “Millimeter Wave Front End,” wherein a same reference oscillator is utilized for various functions. Where the oscillator output is multiplied up to drive the millimeter wave front end as well as used in signal acquisition at an intermediate frequency, a small increase in phase noise at the oscillator may be unacceptable at the radio frequencies and/or intermediate frequencies used. For example, in a system using a 15 MHZ reference frequency multiplied up to provide a 40 GHz mmwave front end, the phase noise of the reference oscillator is magnified over 2500 times, i.e., 20 log (40 GHz/15 MHz)=20 log (2666)=68.5 dB.
Another solution to providing frequency synchronization between two systems in communication is to provide a very precise oscillator, such as may be tuned and calibrated prior to deployment, in each of the systems in order to ensure that the remote unit will always operate within a selected range of the other system. A very narrow band phase lock loop (PLL) may be employed in such a system to accommodate any small amount of drift associated with such oscillators. However, oscillators which may be relied upon to provide such very precise reference frequencies are generally very expensive and, thus, typically do not provide a desirable alternative.
Other solutions have included the use of a dual mode phase lock loop such that the phase lock loop operates in a wide band mode that can acquire the system signal to some degree and, once it locks in at a course range, narrows the loop bandwidth of the phase lock loop. Such a system relies upon the more fine bandwidth of the second mode of the phase lock loop to filter out phase noise. However, experimentation has revealed that the phase noise associated with the wide lock range of the first mode of the dual modes does not provide a reliable lock, particularly at higher frequencies such as millimeter wave frequencies, from which the second mode may operate.
Accordingly, a need exists in the art for systems and methods providing frequency compensation over a relatively large range of frequencies. Moreover, a need exists in the art for such systems and methods to provide such frequency compensation with a very low phase noise associated therewith.
A further need exists in the art for systems and methods providing remote synchronization using frequency compensation techniques to provide accurate frequency synchronization efficiently. Efficiency considerations include not only the cost of components employed in the frequency compensation techniques, but also the ability to minimize the components used and/or to utilize inexpensive components in other portions of the communication system.